The facet joints are a set of synovial, plane joints between the articular processes of two adjacent vertebrae which help support the weight of the body and prevent excessive rotational motion of the spine. These joints are formed by bilateral protrusions of bone of one vertebra that intersect with respective bilateral protrusions of bone of the vertebra located above and the vertebra located below. In particular, superior processes projecting upward from a lower vertebra, their articular surfaces being directed more or less backward (oblique coronal plane), form facet joints with inferior processes projecting downward from a higher vertebra, their articular surfaces being directed more or less forward and outward.
Between each pair of facet joints lies a facet joint capsule composed of cartilage. If the cartilage wears away in the course of consistent motion, the facet joints can become a source of pain in areas ranging from the mid-back to upper-back, the neck to the base of the spine, and in the shoulders. This is referred to as facet joint arthritis or facet arthropathy.
A common and effective treatment for facet joint pain is facet joint fusion. This procedure involves removal of the joint cartilage or drilling a passageway through the joint and placement of a spacer into the joint or passageway in order to restrict the joint's movement and thereby cause bone fusion across the joint. Often, the spacer will take the form of a natural or synthetic bone dowel or bone graft that is inserted into the facet joint. The use of bone dowels presents advantages over metal implants such as screws which are typically inserted transversely through the intersecting facet joint bone protrusions. Advantages include a more efficacious bone fusion and permanent fixation once fusion is achieved.
Implantation of bone dowels between facet joints can be conducted using a minimally invasive procedure which takes place entirely through one or more cannulas inserted through a small incision in a patient's skin and muscle to provide direct access to the joint. Exemplary minimally invasive facet fusion procedures are described in U.S. Pat. Nos. 8,021,392 and 8,231,661. Alternatively, the procedure can be an open or mini-open procedure.
Although use of bone dowels for affecting facet joint fusion and stabilization often succeeds, in some instances dowels will migrate posteriorly out of the drilled passages before fusion can be achieved. This most often occurs if the walls of the passage formed through the joint capsule or joint are smooth or if the dowel fails to include anti-migration features such as fins or ribs. Additionally, undesired dowel migration can occur if the contact surface area between a dowel and the adjacent articular surfaces is too small to overcome the tendency of the dowel to back out of the joint. Accordingly, there is a need for a facet fusion implant configured for preventing undesired, posterior migration of the implant from facet joints.